Method and equipment for rapidly filling a downstream tank with cryogenic liquid from an upstream store

ABSTRACT

The invention relates to a method and equipment for filling at least one downstream tank with a cryogenic liquid, from an upstream store which contains, at a storage pressure above atmospheric pressure, the cryogenic fluid in the liquid phase at the bottom of the tank, and the cryogenic fluid in the gas phase at the top of the tank, said upstream store being suitable for feeding liquid withdrawn from the bottom of the store to the downstream tank, as well as to be supplied with fluid from the outside, characterized in that one generates and maintains a pressure difference between the upstream store and the downstream tank, by pressurizing the gas phase of the upstream store at a pressure that is greater than the equilibrium pressure of the store.

The present invention relates to the field of processes for filling a downstream tank with a cryogenic liquid, such as liquid nitrogen, from an upstream storage tank.

It more particularly relates to processes enabling rapid filling.

Such filling operations are used, for example, to fill the tanks of trucks used to transport and distribute heat-sensitive products such as pharmaceutical products or foodstuffs.

Conventionally, the cryogenic fluid, for example liquid nitrogen, is stored in a high-capacity upstream storage tank connected downstream to equipment that consumes this fluid, such as the tank of a truck, the upstream storage tank containing, under a storage pressure higher than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the tank and in gas phase at the top of the tank, this storage tank being designed to, on the one hand, supply the downstream consuming equipment with liquid extracted from the bottom of the storage tank, and on the other hand, to be supplied from the exterior with fluid.

Most commonly, storage tanks called “low-pressure storage tanks” are used, i.e. the maximum pressure reached at the top of the tank is in general lower than about 4 bar absolute and conventionally the pressure at the top of the storage tank is 1.5 bar relative.

To rapidly transfer the fluid between this upstream storage tank and such a downstream point, for example a tank to be filled, conventionally a cryogenic pump is used to increase the upstream pressure during transfer to the downstream cryogenic tank (see FIG. 1 appended below).

However, it is known that using such cryogenic pumps may entail drawbacks in terms of cost, maintenance, and specific operational constraints such as the pump requiring cooling before use. This is because cryogenic pumps comprise moving parts that require specific maintenance.

Another solution has been suggested, which consists in using an intermediate transfer tank that is pressurized before the ultimate filling of the downstream tank. This solution involves the use of an additional tank, thus entailing a volume constraint and an operating mode that is greatly dependent on the downstream process (pressurization before use and managing filling when it is empty, etc.).

Another solution has been suggested, which consists in maintaining the upstream cryogenic storage tank at the transfer pressure, but it is known, because of the characteristic behavior of cryogenic fluids, that under these conditions the fluid will tend to gravitate toward its equilibrium temperature at the pressure in the storage tank, which will produce a diphasic fluid during transfer and therefore reduce the flow rate because there will be gas in the flow (by way of illustration, 1% of diphasic fluid by weight in nitrogen implies a ratio of the mass of gas to the total mass equivalent to a void rate, i.e. the volume occupied by the gas/total volume, of 50%).

One of the objectives of the present invention is thus to provide a novel approach to rapid filling, solving the technical problems described above.

As will be seen in more detail below, the present invention provides a new filling process, the essential features of which may be summarized as follows:

-   -   the upstream storage tank is used directly, without an         intermediate additional tank;     -   a pressure difference is created between the upstream storage         tank and the downstream point, by establishing an “overpressure”         in this upstream storage tank by pressurizing it to a pressure         higher than the equilibrium pressure corresponding to the         temperature of the cryogenic fluid in the storage tank, and by         maintaining, via control/regulation means, such a higher         pressure, thereby allowing the subcooled liquid to be         transferred at its transfer pressure and therefore allowing         filling operations to be carried out at higher flow rates, by         limiting, during the transfer, losses due to vaporization,         related to head loss in the installation, and to the ingress of         heat (the time it takes for the temperature of the liquid to         increase being sufficiently long relative to the time it stays         in the upstream storage tank between two filling operations);     -   to do this, according to a preferred embodiment of the         invention, a control device is fitted to or associated with the         upstream storage tank, said device being able to automatically         manage the supply pressure of the cryogenic liquid in the         upstream storage tank at the foot of the tank (liquid phase),         and the temperature of the stored cryogenic liquid, so as to         maintain the gas phase in this upstream storage tank at such a         pressure that is higher than the equilibrium pressure in the         storage tank.

The present invention thus relates to a process for filling at least one downstream tank with a cryogenic liquid from an upstream storage tank, which upstream storage tank contains, under a storage pressure higher than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the storage tank and in gas phase at the top of the storage tank, said upstream storage tank being designed to supply the downstream tank with liquid extracted from the bottom of the storage tank, and to be supplied from the exterior with fluid, noteworthy in that a pressure difference is created and maintained between the upstream storage tank and the downstream tank, by establishing, in the gas phase in the upstream storage tank, a pressure that is higher than the equilibrium pressure in the storage tank.

According to one embodiment of the invention, the gas phase in the upstream storage tank is brought to and maintained at a pressure equal to:

P_(g)=ΔP+P₁−pgh

where:

-   -   P_(g) is the pressure maintained in the gaseous atmosphere in         the storage tank;     -   ΔP is the head loss in the line between the upstream storage         tank and the downstream tank;     -   P₁ is the pressure of the liquid in the line;     -   p is the density of the stored liquid;     -   g=9.81 m/s²; and     -   h is the height of liquid available.

Other features and advantages of the present invention will become more clearly apparent from the following description, given by way of completely nonlimiting illustration, and with regard to the appended drawings, in which:

FIG. 1 is a partial schematic view of a conventional installation for filling a downstream tank with liquid nitrogen from an upstream storage tank, using a cryogenic pump; and

FIG. 2 is a partial schematic view of a rapid filling installation according to the invention.

FIG. 1 shows the conventional structure of such a filling installation, a pressurizing means for pressurizing the liquid extracted from the foot of the storage tank and transported to the downstream tank being present on the line, which structure is well known and will therefore not be described further here.

For its part, FIG. 2 illustrates an installation according to the invention, which does not comprise such a pressurizing means, but in contrast comprises a control device able to act automatically on the supply pressure of the cryogenic liquid at the foot of the upstream storage tank and on the temperature of the stored cryogenic liquid, in response to a measurement of the pressure of the gas phase in the upstream storage tank and the position of this measurement relative to the equilibrium pressure in the storage tank, so as to maintain a pressure difference between the upstream storage tank and the downstream tank and to maintain this gas-phase pressure at a level above the equilibrium pressure of the storage tank, the equilibrium pressure corresponding to the temperature of the cryogenic fluid in the storage tank.

By way of example, the transfer pressure of the cryogenic fluid is set to the nominal desired value, for example 5 bar relative for a given downstream application using liquid nitrogen, the temperature of the fluid is moreover controlled relative to a reference value under given conditions, for example −187° C., which corresponds to an equilibrium pressure of 1.5 bar relative. If after a period of use, the temperature difference with respect to the setpoint value is greater than the allowed hysteresis, a new pressure setpoint is delivered to the controller in order to reduce the pressure of the associated gaseous atmosphere and therefore limit heating of the fluid in the storage tank during idle periods during the transfers. The system resets to the pressure setting with regard to operational reuse of the rapid transfer installation. The control device thus enables parameterized control of pressure, temperature and time data in order to optimize the overall consumption of the installation.

The table of experimental results below will allow the advantages of such conditions, according to the invention, to be better understood.

This table indicates the filling times obtained for the transfer of 410 liters of liquid nitrogen from upstream to downstream, for various operating conditions:

-   -   upstream to downstream pressure deltas of 1, 2 or 3 bar;     -   in combination with temperatures of the liquid phase in the         upstream storage tank regulated to −177° C., −181, −187 or −191°         C.;     -   in each case, the value “P_(sat)” represents the equilibrium         pressure of the nitrogen at the liquid temperature considered.

For each set of operational conditions, the table gives the filling time and the average flow rate achieved.

Thus, by way of example, for a delta of 3 bar (P_(upstream)=5 bar and P_(downstream)=2 bar) with the temperature of the liquid regulated to −187° C., the 410 liters of a downstream tank were filled in 3.7 minutes, with an average flow rate of 110.8 1/min.

TABLE 1 Pressure Filling time for 410 liters and average flow rate achieved difference T_(|) = −191° C. T_(|) = −187° C. T_(|) = −181° C. T_(|) = −177° C. (bar) Psat = 0.7 bar Psat = 1.5 bar Psat = 3.2 bar Psat = 4.8 bar 1 6.6 mn 7.3 mn 8.3 mn 9 mn 62.1 l/mn 56.2 l/mn 49.4 l/mn 45.6 l/mn 2 4 mn 4.8 mn 6.2 mn 7.3 mn 102.5 l/mn 85.4 l/mn 66.1 l/mn 56.2 l/mn 3 3 mn 3.7 mn 5.2 mn 6.4 mn 136.7 l/mn 110.8 l/mn 78.8 l/mn 64.1 l/mn

The following teachings may be deduced from this table:

-   -   columns 3 and 4 (−181° C., −177° C.) show a striking reduction         in flow rate (comparative examples);     -   the tests of column 1 are in accordance with the invention, but         it could be said that they represent an implementation that,         while certainly being possible, is more expensive;     -   the tests of column 2 (−187° C., pressure delta of 2 or 3 bar)         represent a very good compromise between performance and         implementation cost for the application and installation         supplied here. 

1-4. (canceled)
 5. A process for filling at least one downstream tank with a cryogenic liquid from an upstream storage tank, the upstream storage tank contains, under a storage pressure higher than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the storage tank and in gas phase at the top of the storage tank, said upstream storage tank being designed to supply the downstream tank with liquid extracted from the bottom of the storage tank, and to be supplied from the exterior with fluid, the process comprising creating and maintaining a pressure difference between the upstream storage tank and the downstream tank, by establishing, in the gas phase in the upstream storage tank, a pressure that is higher than the equilibrium pressure in the storage tank.
 6. The filling process of claim 5, wherein the pressure of the gas phase in the upstream storage tank is maintained by acting on the supply pressure of the cryogenic liquid in the upstream storage tank at the foot of the tank and on the temperature of the stored cryogenic liquid.
 7. The filling process of claim 6, wherein a control device is provided, able to automatically manage the supply pressure of the cryogenic liquid in the upstream storage tank at the foot of the tank and the temperature of the stored cryogenic liquid, in response to a measurement of the pressure of the gas phase in the upstream storage tank and the position of this measurement relative to the equilibrium pressure in the storage tank.
 8. An installation for filling at least one downstream tank with a cryogenic liquid from an upstream storage tank, which upstream storage tank contains, under a storage pressure higher than atmospheric pressure, the cryogenic fluid in liquid phase at the bottom of the storage tank and in gas phase at the top of the storage tank, said upstream storage tank being designed to supply the downstream tank with liquid extracted from the bottom of the storage tank, and to be supplied from the exterior with fluid, comprising a control device able to automatically manage the supply pressure of the cryogenic liquid in the upstream storage tank at the foot of the tank and the temperature of the stored cryogenic liquid, in response to a measurement of the pressure of the gas phase in the upstream storage tank and the position of this measurement relative to the equilibrium pressure in the storage tank, so as to maintain, in the gas phase in the upstream storage tank, a pressure that is higher than the equilibrium pressure in the storage tank. 